Mikel Arteta's side took the lead in the second half through Leandro Trossard, but were pegged back by Ivan Toney's equaliser and they were unable to get a late winner against a resolute Brentford.
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Thomas Frank's team are one of the most in-form sides in the Premier League and carved several good chances of their own, doing well against the league leaders.
And Agbonlahor feels the Gunners are lacking that edge to take them to the title, with the former Aston Villa forward suggesting Arsenal are missing Gabriel Jesus, who has not played since before the World Cup due to injury.
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Speaking on GameDay Phone-In, Agbonlahor said: "Arsenal today, and last week against Everton, this is the first time I'm starting to think that they don't look as convincing.
"Today, it looked quite easy for Brentford to stop Arsenal. It's a bit of a worry for Arsenal and Mikel Arteta needs Gabriel Jesus back.
"The scary thing for Arsenal fans is that Manchester City can be on top of the league on goal difference if they win their games tomorrow against Aston Villa and against Arsenal on Wednesday.
"That's a scary thought for the fans that have been so confident.
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"Players have got to turn up. Martinelli has been the same the past few weeks. Trossard came on and scored and he did well.
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"It's just looking like Arsenal now are not looking convincing for me. I think Manchester City will come back.
Arsenal still have to play Manchester City both home and away and the reigning champions can take the lead in the table if they manage to win both their games.
The Gunners haven't won the Premier League since 2004 and haven't played Champions League football since 2017, and will be hoping to bring those runs to an end in this season.